ORCA™ – Conductive AFM Imaging
Using the MFP-3D™ AFM

AFM

Asylum Research

Conductive AFM is a powerful current sensing technique for electrical characterization of conductivity variations in resistivesamples. It allows current measurements in the range of hundreds of femtoamps to ten microamps. Conductive AFM cansimultaneously map the topography and current distribution of a sample. It is a measurement useful in a wide variety of material
characterization applications including thin dielectric films, ferroelectric films, nanotubes, conductive polymers, and others.

How It WorksThe ORCA module consists of a specially-designed cantilever
holder that includes a transimpedance amplifier. The gainof the amplifier can be chosen by the user. Standard values
range from 5 x 107 to 5 x 109 volts/amp. The cantilever
holder is used with conductive AFM probes to make themeasurement. The easiest technique for measuring thelocalized conductivity of a sample is to combine the current
measurements with contact mode AFM imaging. All images
in this application note were acquired using contact modewith a PtIr coated Electri-Lever (Olympus), with a nominal
spring constant of 1-2 N/m and good wear characteristics.
Note that coated cantilevers are vulnerable to imaging
artifacts associated with irreversible changes in the tip
shape or coating. This is an important consideration wheninterpreting ORCA measurements.

ORCA Sample Mount (left), and Cantilever Holder (right).

Current Measurements
Data in this application note was made using a gain of 5 x 108
volts/amp on the initial stage (see ORCA-58 in Figure 1).
On the MFP-3D, the ORCA output was digitized with oneof the auxiliary 100 kHz ADCs and then digitally filtered at1 kHz. The measured RMS noise for these settings was
0.5 pA, consistent with the Johnson noise performance
predicted in Figure 1.
Figure 2 shows an example image made at a 1.5 volt bias.
The sample is a 10 nm thick film of Europium doped ZnO.
This is a relatively high resistivity sample, particularly
challenging for conductive AFM measurements. The contact
mode topographic image “A” shows a relatively uniform
grainy structure. The current image “B”, however, shows
patches of high conductivity surrounded by very low
conductivity regions (see Figure 3 for a higher resolution scan).
The NPS™ nanopositioning closed loop sensors on theMFP-3D make it possible to reproducibly position thecantilever at a point of interest as shown by the colored circles
in Figure 2B. The tip was positioned in the center of thecolored circles using the MFP-3D’s “pick a point” force curveinterface. The bias voltage was then swept from -5 to 5 voltsand the response current measured. Figure 2C shows
the resulting current-voltage (IV) curves. The conductivity
curves are consistent with the contrast observed in Figure 2B.
Specifically, the conductivity is highest at the position
marked with the black circle, in between at the red,and lowest at the blue.

1Figure 1: Gain selection chart. Johnson noise and the relevant current
ranges for a transimpedance amplifier digitized at 16 bits. At a gainof nearly 1010 volts/amp, Johnson noise is equivalent to the best
resolution of a 16-bit ADC. At smaller gains, the main limitation isthe resolution of the ADC. At higher gains, Johnson noise dominates.
AB

C

Figure 2: Topography (A) and current (B) image of a Europium-dopedZnO sample at a bias of 1.5 volts, 2 µm scan sample courtesy ofthe Krishnan Lab, Univ. of Washington. Corresponding IV curves(C) recorded at three specific positions indicated in B. The curves
are consistent with the current contrast observed in 2B. Specifically,
the conductance is highest at the black location, medium at thered and lowest at the blue.

Current as a Function of Loading ForceUsing the flexible IGOR Pro software interface and the ARC2™
Controller, ORCA can also be used for a variety of transportmeasurements. Figure 4 shows the effects of loading force onthe IV curves. For this experiment, the cantilever was positioned
at an XY point on the sample. The cantilever deflection wasmaintained at a constant value with a feedback loop. At thesame time, the tip bias was swept sinusoidally at an amplitudeof 1 volt and a frequency of 10 Hz. The measured current wasthen plotted vs. the drive voltage (see Figure 4). Not surprisingly,
as the loading force is increased, the conductance increases.
There is also more noise in the signal at smaller loading forces.

Figure 3: High resolution topography (l) and current (r) at a bias of
1.5 volts, 50 pA scale, 2 µm scan. Sample courtesy K. Krishnan Lab,University of Washington.

Combined Force and Current Measurements
Figure 5 shows another measurement with the current flow as afunction of the load at two fixed biases. There is an asymmetry
in the responses at the different biases consistent with the IV
curve measurements in Figures 2C and 4. There are also some
additional features in these curves. First, the conductance seems
to be highest at a point where there is a step in the contact
portion of the force curve. There is also a difference between theextension and retraction curves. These features are consistent
with the contact resistance of the sample varying as a functionof load, although other explanations are certainly plausible.

Specifications

Figure 4: IV curves as a function of loading force on a Europium doped
ZnO sample. Each curve was recorded at the same X-Y position on thesample as a function of cantilever load.

l T he standard ORCA cantilever holder has a gain of 5 x 108
volts/amp (~1 pA to 20 nA).
l The Dual Gain ORCA cantilever holder has gains of 1 x 106
and 1 x 109 volts/amp (~1 pA to 10 µA).
l O ther gain values are available.
l Includes cantilever holder, sample holder, and test resistor.

Conclusion
The ORCA conductive AFM option for the MFP-3D provides
low-noise, flexible transport measurements at the nanoscale.
The flexible software environment enables a variety of standard
measurements to be made, as well as allowing the researcher
to define their own experiments.

Figure 5: Current (above) and force (below) as the cantilever extends
towards the ZnO sample surface and retracts away. The blue curves
show the positively biased response while the red shows the negatively
biased response.

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The foregoing application note is copyrighted by Oxford Instruments Asylum Research, Inc. Oxford Instruments Asylum Research, Inc.
does not intend the application note or any part thereof to form part of any order or contract or regarded as a representation relating
to the products or service concerned, but it may, with acknowledgement to Oxford Instruments Asylum Research, Inc., be used, appliedor reproduced for any purpose. Oxford Instruments Asylum Research, Inc. reserves the right to alter, without notice the specification,
design or conditions of supply of any product or service. Application Note 2 – 6/2014.

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